docs: Add C++ unordered_set::clear() term entry (#8030)

[Rishabh1925]

Closes #8030. This PR adds the documentation entry for the C++ unordered_set::clear() method. The entry explains that the method empties the set while generally preserving the container's internal capacity. Includes the mandatory Syntax, Example, and Codebyte sections.

[Rishabh1925]

Hello @mamtawardhani @codecademy-docs

I've finished creating the new term entry for unordered_set::clear(). The documentation clarifies that the size becomes zero, and the set remains usable for new elements. Ready for review!